Feedback

Feedback is an alteration of the input to a system based upon a monitored internal state that changes the input either to modify that state (positive feedback), or to maintain it (negative or degenerative feedback). Unlike feedforward control, which anticipates a need for change, feedback introduces change only as a response to a disturbance.

Examples

(PD) Diagram: John R. Brews
Keeping a car at the speed limit.

(PD) Diagram: John R. Brews
Thermostatic control of house temperature using a negative feedback system.

An everyday example of negative feedback to control a system is shown at the left, where a car is maintained at the speed limit by a driver using the throttle (accelerator) to adjust to road conditions (for example, curves or grades). The car's speed is decided by a combination of the road conditions and the gas supplied, and the gas supplied is the system feedback, the amount of which is controlled by the driver who compares the speedometer reading with the speed limit.

A more formal example of negative feedback is the thermostatic control of house temperature. In the figure, the house temperature is a response to the combined heat input from the world and from the furnace. Suppose a change occurs in the heat supplied to the house by the outside world. The resultant house temperature is sensed by a thermometer and converted to an electrical signal. This signal is compared by the thermostat to a set-point signal that corresponds to the desired temperature value. The discrepancy between the set point and the signal is calculated as an error signal by the thermostat, triggering (activating) action by the furnace. Depending upon the sign of the error the furnace produces more or less heat, regulating the combined heat input to the house so the temperature stays constant. In other words, the information on system temperature is used to generate feedback that keeps the temperature at value.

It might be noted that translation of signals between various forms (such as temperature, chemical signals, electrical signals, mechanical signals) occurs throughout the feedback loop. Also, there are time delays introduced at each step in a feedback loop that affect its success in tracking variations in the world's input.

The same principle is found in biology as homeostasis.

Terminology

To discuss the general example, some formal terminology that does not depend upon the specific system is useful.

• Control variable: The monitored parameter used to determine the system status; for example, speed of the car or temperature of the house.
• Measured variable: The translated form of the control variable as output from a sensor. For example, the speedometer reading of vehicle speed, or the thermometer reading corresponding to house temperature.
• Set point: The value of the measured variable that is considered desirable.
• Error: The discrepancy between the measured and set-point values as calculated by a controller; for example, the driver's observation of the discrepancy between speed and speed limit, or the output of the thermostat.
• Manipulated variable: The feedback variable to the system; for example the throttle position controlling gasoline input to the car or the gas valve position controlling fuel input to the furnace.
• Load variable: The environmental input affecting the system; for example the grade of the road or the heat from the sun.
• Controller: The mechanism that detects the discrepancy between the set point and the actual system status.
• Sensor: The mechanism used to translate the system status to a form understood by the controller.
• Effector: The mechanism supplying the feedback response to the system. In part, the carburetor and fuel supply system in the car, or the furnace in the house.
• Receptor: The part of the system subject to control; for example, the car or the house.